More than thirty years ago, many household laundry detergents were made of branched alkylbenzene sulfonates (BABS). BABS are manufactured from a type of alkylbenzenes called branched alkylbenzenes (BAB). Alkylbenzenes (phenyl-alkanes) refers to a general category of compounds having an aliphatic alkyl group bound to a phenyl group and having the general formula of (m.sub.i -alkyl.sub.i).sub.i -n-phenyl-alkane. The aliphatic alkyl group consists of an aliphatic alkyl chain, which is referred to by "alkane" in the (m.sub.i -alkyl.sub.i).sub.i -n-phenyl-alkane formula. Of the chains of the aliphatic alkyl group, the aliphatic alkyl chain is the longest straight chain that has a carbon bound to the phenyl group. The aliphatic alkyl group may also consist of one or more alkyl group branches, each of which is attached to the aliphatic alkyl chain and is designated by a corresponding "(m.sub.i -alkyl.sub.i).sub.i " in the (m.sub.i -alkyl.sub.i).sub.i -n-phenyl-alkane formula. If it is possible to select two or more chains of equal lengths as the aliphatic alkyl chain, the choice goes to the chain carrying the greatest number of alkyl group branches. The subscript counter "i" thus has a value of from 1 to the number of alkyl group branches, and for each value of i, the corresponding alkyl group branch is attached to carbon number m.sub.i of the aliphatic alkyl chain. The phenyl group is attached to the aliphatic alkyl group, specifically to carbon number n of the aliphatic alkyl chain. The aliphatic alkylation chain is numbered from one end to the other, the direction being chosen so as to give the lowest number possible to the position of the phenyl group.
The standard process used by the petrochemical industry for producing BAB consists of oligomerizing light olefins, particularly propylene, to branched olefins having 10 to 14 carbon atoms and then alkylating benzene with the branched olefins in the presence of a catalyst such as HF. Although the product BAB comprises a large number of alkyl-phenyl-alkanes having the general formula (m.sub.i -alkyl.sub.i).sub.i -n-phenyl-alkane, for the purpose of illustrating three important characteristics of BAB it is sufficient to point out only two examples of BAB: m-alkyl-m-alkyl-n-phenyl-alkanes where m.noteq.n, and m-alkyl-m-phenyl-alkanes where m.gtoreq.2.
The most prominent characteristic of BAB is that, for a large proportion of BAB, there is attached to the aliphatic alkyl chain of BAB generally at least one alkyl group branch, and more commonly three or more alkyl group branches. BAB thus has a relatively large number of primary carbon atoms per aliphatic alkyl group, since the number of primary carbon atoms per aliphatic alkyl group in BAB equals the number of alkyl group branches per aliphatic alkyl group plus either one if n=1, or two if n.gtoreq.2, provided that the alkyl group branches themselves are unbranched. If any alkyl group branch itself is branched, then the aliphatic alkyl group in BAB has even more primary carbon atoms. Thus the aliphatic alkyl group in BAB usually has three, four, or more primary carbon atoms. As for the alkyl group branches of the aliphatic alkylation group in BAB, each alkyl group branch is usually a methyl group branch, although ethyl, propyl, or higher alkyl group branches are possible.
Another characteristic of BAB is that the phenyl group in BAB can be attached to any non-primary carbon atom of the aliphatic alkyl chain. This is typical of BAB that is produced from the standard BAB process used by the petrochemical industry. Except for 1-phenyl-alkanes whose formation is known to be disfavored due to the relative instability of the primary carbenium ion and neglecting the relatively minor effect of the branches of the branched paraffins, the oligomerization step produces a carbon-carbon double bond that is randomly distributed along the length of the aliphatic alkyl chain, and the alkylation step nearly randomly attaches the phenyl group to a carbon along the aliphatic alkyl chain. Thus, for example, for a phenyl-alkane which has an aliphatic alkyl chain having 10 carbon atoms and which was produced by the standard BAB process, the phenyl-alkane product would be expected to be an approximately random distribution of 2-, 3-, 4-, and 5-phenyl-alkanes, and the selectivity of the process to a phenyl-alkane like 2-phenyl alkane would be 25 if the distribution was perfectly random, but is typically between about 10 and about 40.
A third characteristic of BAB is the relatively high probability that one of the carbons of the aliphatic alkyl group is a quaternary carbon. In BAB, the quaternary carbon may be, as illustrated by the first BAB example, a carbon in the aliphatic alkyl group other than the carbon that is bonded by a carbon-carbon bond to a carbon in the phenyl group. However, as is illustrated by the BAB second example, the quaternary carbon may also be the carbon that is bonded by a carbon-carbon bond to a carbon in the phenyl group. When a carbon atom on the alkyl side chain not only is attached to two other carbons on the alkyl side chain and to a carbon atom of an alkyl group branch but also is attached to a carbon atom of the phenyl group, the resulting alkyl-phenyl-alkane is referred to as a "quaternary alkyl-phenyl-alkane" or simply a "quat." Thus, quats comprise alkyl-phenyl-alkanes having the general formula m-alkyl-m-phenyl-alkane. If the quaternary carbon is the second carbon atom numbered from an end of the alkyl side chain, the resulting 2-alkyl-2-phenyl-alkane is referred to as an "end quat." If the quaternary carbon is any other carbon atom of the alkyl side chain, as in the second BAB example, then the resulting alkyl-phenyl-alkane is referred to as an "internal quat." In known processes for producing BAB, a relatively high proportion, typically greater than 10 mol-%, of the BAB is internal quats.
About thirty years ago it became apparent that household laundry detergents made of BABS were gradually polluting rivers and lakes. Investigation into the problem led to the recognition that BABS were slow to biodegrade. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than BABS. Today, detergents made of LABS are manufactured worldwide. LABS are manufactured from another type of alkylbenzenes called linear alkylbenzenes (LAB). The standard process used by the petrochemical industry for producing LAB consists of dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of a catalyst such as HF or a solid catalyst. LAB are phenyl-alkanes comprising a linear aliphatic alkyl group and a phenyl group and have the general formula n-phenyl-alkane. LAB has no alkyl group branches, and consequently the linear aliphatic alkyl group normally has two primary carbon atoms (i.e., n.gtoreq.2). Another characteristic of LAB that is produced by the standard LAB process is that the phenyl group in LAB is usually attached to any secondary carbon atom of the linear aliphatic alkyl group. In LAB produced using HF catalyst the phenyl group is slightly more likely to attach to a secondary carbon near the center as opposed to near the end of the linear aliphatic alkyl group, while in LAB produced by the Detal.TM. process approximately 25-35 mol-% of n-phenyl-alkanes are 2-phenyl-alkanes.
Over the last few years, other research has identified certain modified alkylbenzene sulfonates, which are referred to herein as MABS, which are different in composition from all alkylbenzene sulfonates used currently in commerce, including BABS and LABS, and from all alkylbenzene sulfonates produced by prior alkylbenzene processes, including those which alkylate aromatics using catalysts such as HF, aluminum chloride, silica-alumina, fluorided silica-alumina, zeolites, and fluorided zeolites. MABS also differ from these other alkylbenzene sulfonates by having improved laundry cleaning performance, hard surface cleaning performance, and excellent efficiency in hard and/or cold water, while also having biodegradability comparable to that of LABS.
MABS can be produced by sulfonating a third type of alkylbenzenes called modified alkylbenzenes (MAB), and the desired characteristics of MAB are determined by the desired solubility, surfactancy, and biodegradability properties of MABS. MAB is a phenyl-alkane comprising a lightly branched aliphatic alkyl group and a phenyl group and has the general formula (m.sub.i -alkyl.sub.i).sub.i -n-phenyl-alkane. MAB usually has only one alkyl group branch, and the alkyl group branch is a methyl group, which is preferred, an ethyl group, or an n-propyl group, so that, where there is only one alkyl group branch and n.noteq.1, the aliphatic alkyl group in MAB has three primary carbons. However, the aliphatic alkyl group in MAB may have two primary carbon atoms if there is only one alkyl group branch and n=1, or, if there are two alkyl group branches and n.noteq.1, four primary carbons. Thus, the first characteristic of MAB is that the number of primary carbons in the aliphatic alkyl group in MAB is intermediate between that in BAB and that in LAB. Another characteristic of MAB is that it contains a high proportion of 2-phenyl-alkanes, namely that from about 40 to about 100% of phenyl groups are attached selectively to the second carbon atom as numbered from an end of the alkyl side chain.
A final characteristic of the MAB alkylate is that the MAB has a relatively low proportion of internal quats. Some internal quats such as 5-methyl-5-phenyl-undecane produce MABS that has shown slower biodegradation, but end quats such as 2-methyl-2-phenyl-undecane produce MABS that show biodegradation similar to that of LABS. For example, biodegradation experiments show that in a porous pot activated sludge treatment, the ultimate biodegradation was greater for sodium 2-methyl-2-undecyl [C.sup.14 ] benzenesulfonate than for sodium 5-methyl-5-undecyl [C.sup.14 ] benzenesulfonate. See the article entitled "Biodegradation of Coproducts of Commercial Linear Alkylbenzene Sulfonate," by A. M. Nielsen et al., in Environmental Science and Technology, Vol. 31, No. 12, 3397-3404 (1997). A relatively low proportion, typically less than 10 mol-%, of MAB is internal quats.
Because of the advantages of MABS over other alkylbenzene sulfonates, catalysts and processes are sought that selectively produce MAB. As suggested by the foregoing, two of the chief criteria for an alkylation process for the production of MAB are selectivity to 2-phenyl-alkanes and selectivity away from internal quaternary phenyl-alkanes. Prior art alkylation processes for the production of LAB using catalysts such as aluminum chloride or HF are incapable of producing MAB having the desired 2-phenyl-alkane selectivity and internal quat selectivity. In these prior art processes, when lightly branched olefins (i.e., olefins that have essentially the same light branching as that of the aliphatic alkyl group of MAB) react with benzene, quaternary phenyl-alkanes selectively form. One reaction mechanism that accounts for such selective quaternary phenyl-alkane formation is that the delinearized olefins convert, to various extents, into primary, secondary, and tertiary carbenium ion intermediates. Of these three carbenium ions, tertiary carbenium ions are the most stable, and because of their stability, are the most likely to form and react with benzene, thus forming a quaternary phenyl-alkane.
One process that has been proposed for producing MAB comprises a three-step process. First, a feedstock comprising paraffins is passed to an isomerization zone to isomerize the paraffins and to produce an isomerized product stream comprising lightly branched paraffins (i.e., paraffins that have essentially the same light branching as that of the aliphatic alkyl group of MAB). Next, the isomerized product stream passes to a dehydrogenation zone where the lightly branched paraffins are dehydrogenated to produce a dehydrogenated product stream comprising lightly branched monoolefins (i.e., monoolefins that have essentially the same light branching as that of the lightly branched paraffins, and, consequently, that of the aliphatic alkyl group of MAB). Finally, the dehydrogenated product stream passes to an alkylation zone where the lightly branched monoolefins in the dehydrogenated product stream react with benzene to form MAB.
One of the problems with this proposed process is that conventional dehydrogenation reaction zones typically convert only about 10 wt-% of the entering paraffins to olefins, so that usually about 90 wt-% of the product stream from the dehydrogenation zone comprises paraffins, including both linear and nonlinear paraffins. Because the product stream from the dehydrogenation zone enters the alkylation zone, these paraffins all enter the alkylation zone as well. Although it would be desirable to remove the paraffins prior to entering the alkylation zone, the difficulty of separating these paraffins from the monoolefins all of the same carbon number precludes such an arrangement. In the alkylation zone, typically more than 90 wt-% of the entering monoolefins are converted to phenyl-alkanes while the entering paraffins are essentially inert or unreactive. Thus, the alkylation effluent contains not only the desired product MAB but also these paraffins. Accordingly, processes for the production of MAB are sought that efficiently recover and utilize paraffins in the alkylation effluent.